The development of antibody forming cells (AFC) is being studied in tissue culture with a new system. A low density subpopulation of primed mouse spleen cells is boosted in vitro with specific antigen in the presence of isologous serum. Large and highly enriched populations of antigen dependent AFC develop. The AFC precursors can be radiolabeled initially and are found to undergo several divisions in vitro during their development into AFC. Moreover, the proliferating cells associate physically with a novel dendritic cell whose function is still unclear. This system is being used to study, more directly than previously possible, the effects of specific antigen and various accessory cells on the proliferation and maturation of AFC.